Increasing the Power: Absorption Bleach, Thermal Quenching, and Auger Quenching of the Red‐Emitting Phosphor K₂TiF₆:Mn⁴⁺

Abstract

This does not only create a warmer white color but also generally raises the color rendering index, thereby making the color of the lamp more pleasant to the human eye. [5] However, it comes with the downside of decreased luminous efficacy due to emission at wavelengths where the eye is less sensitive. [6] An alternative class of red-emitting materials with higher luminous efficacy than CASN:Eu 2+ , is the Mn 4+ -doped fluorides. [7][8][9] These materials have absorption bands in the ultraviolet (UV) and blue due to spin-allowed transitions. Their roomtemperature wet-chemical synthesis is relatively simple and the narrow emission lines around 630 nm give rise to a red color with a high luminous efficacy. [10,11] These properties make Mn 4+ -doped fluorides excellent blue-to-red converting phosphors for low-power applications such as displays. However, for lighting applications where the light powers are generally orders of magnitude higher, the use of Mn 4+ -doped fluorides is complicated by droop: at increasing blue illumination powers, the output of the phosphor increases sub-linearly, approaches a maximum, and may eventually even drop. [12][13][14] These phenomena can occur already at excitation densities of 100 W cm −2 that are typical in (home) lighting w-LEDs and negatively affect the energy efficiency and the perceived color of Mn 4+ -containing w-LEDs. [15] The origin of luminescence droop of Mn 4+ -doped fluorides is not entirely clear and various mechanisms have been proposed. For example, Mn 4+ -doped fluorides are known to suffer from temperature quenching above temperatures of 400-500 K. [16] Illumination-induced heating may hence lead to an efficiency drop at high excitation powers. [17] In addition, the long 5-10 ms lifetime of the 2 E excited state of Mn 4+ could contribute to droop as it results in a high steady-state population of excited Mn 4+ ions. Excited Mn 4+ ions bleach the 4 A 2 → 4 T 2 absorption, which ultimately limits the photon conversion rate per Mn 4+ dopant to an amount equal to the inverse of the lifetime. More indirectly, the high steady-state population of excited Mn 4+ at high excitation powers may lead to additional losses through excited-state absorption of the blue excitation light or through Auger energy transfer. [18] These losses could be remedied in part by gentle illumination-induced heating up to 400 K because the radiative lifetime and, hence, the steady-state 2 E excited state population decrease with temperature without the loss of light output. [19] Mn 4+ -doped fluorides are popular phosphors for warm-white lighting, converting blue light from light-emitting diodes (LEDs) into red light. However, they suffer from droop, that is, decreasing performance at increasing power, limiting their applicability for high-power applications. Previous studies highlight different causes of droop. Here, a unified picture of droop of Mn 4+doped K 2 TiF 6 , accounting for all previously proposed mechanisms, is provided. Combining continuous-wave and pulsed experiments on samples o...

High-Efficiency Mn4+-Doped Rb2GeF6 Single-Crystal Phosphors: Enhanced Moisture Resistance and Performance in Warm White LEDs

“…The normalized PL intensities indicate that RGFMC and RGFMP exhibit excellent thermal stability. 33,38 The drop in PL can be attributed to elemental impurities causing quenching through energy transfer or metal-to-metal charge transfer in typical phosphors. Additionally, increased backscattering and self-absorption of the phosphor particles lead to higher nonradiative losses.…”